Space turbulence recreated on tabletop

Our experiments involved the excitation of a solid surface with high-energy laser pulses, each pulse lasting a few tens of femtoseconds, and popularly known as “ultrafast light”.

Published:July 10, 2017 12:40 am
High-power laser used to recreate astrophysical turbulence in TIFR lab

G Ravindra Kumar & team, Tata Institute of Fundamental Research, Mumbai

Amita Das & teaM, Institute for Plasma Research, Gandhinagar 

One of the several different features of outer space that scientists have been keenly studying in the hope of further unlocking secrets of the universe, is the all-pervasive phenomena of magnetic field turbulence. Magnetic fields are present everywhere — in the planets, stars, galaxies, clouds of gases. Very often, these magnetic fields are disordered, chaotic and irregular. There are sharp fluctuations in the strength of the magnetic fields and abrupt breaks. Scientists refer to these as magnetic field turbulence, similar to the turbulence of ocean currents or the dispersal of smoke from an industrial chimney.

Scientists do not fully understand the causes and nature of this turbulence in magnetic fields. Nevertheless, they believe this turbulence holds important insights into the way the universe was formed and behaves. We at TIFR in Mumbai and IPR in Gandhinagar have been interested for a long time in studies of matter at high energy density. Experiments at TIFR have been attempting to create extreme conditions of temperature and density that somewhat mimic the conditions in outer space, while IPR theorists have been trying to model these systems and simulate the results. Recently, we produced breakthrough results that we believe can help us better understand the nature of magnetic field turbulence.

Our experiments involved the excitation of a solid surface with high-energy laser pulses, each pulse lasting a few tens of femtoseconds, and popularly known as “ultrafast light”. The powerful burst of energy within a very short time (a femtosecond is a thousandth of a trillionth of a second) ensures a substantial increase in temperature by excitation of ionised electrons. However, the positive ions in the solid, being much heavier than electrons, do not get sufficiently excited within the same time-span. These require greater exposure times to get excited, something of the order of 1,000s of femtoseconds. But that kind of exposure is not available. So what we see is a sudden rise in temperature but practically zero thermal expansion. The density of the solid does not change during the excitation by the laser pulse, unlike in normal circumstances where a solid expands on slow heating. At the end of all this excitation, we have hot, dense ionised gas called “plasma” on the solid surface.

For our experiment, we applied the classic “pump & probe approach”. Each laser pulse was split into two. The first one would excite the electrons while the second, delayed from the first and reduced in energy, would observe the effects of the exposure to the first pulse. Repeated pulses were used, each irradiating a new portion on the solid surface. The effect of exposure to the very high-power laser pulses was the induction of a giant current in the solid — owing to the excitation of electrons and their movement — though for a very brief time. The current in turn creates a huge magnetic field, of the order of millions of Gauss. Just to put things in perspective, the earth’s magnetic field is just about one Gauss.

When we studied these magnetic fields, we noticed the presence of turbulence. The surprising revelation was that the characteristics of this turbulence were very similar to what satellite data have been telling us about the magnetic field turbulence in outer space. The spectral features of the magnetic-field energy density were observed to be very similar. This is a major surprise because the origins of the turbulence in magnetic fields in the two fields are very different. It has led us to propose that the spectral properties of magnetic field turbulence are actually independent of the source creating the turbulence. The Instituto Superior Technico, Universidade de Lisboa, Portugal collaborated with us in this work.

What is really exciting is that our results further the possibility of designing tabletop experiments to study the processes occurring far away in distant planets and stars. These have the potential to improve the understanding of the phenomena of magnetic field turbulence. We published our findings in Nature Communications that went online on June 30. This piece is also a tribute to Prof Predhiman Kaw, pioneering plasma physicist, “father of Indian nuclear fusion efforts”, great educator and a generous mentor who provided much of the vision for our research over the last nearly 20 years. He co-authored this paper with us but unfortunately passed away a couple of weeks before it was published.

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